El Castillo San Felipe del Morro (Fort El Morro), San Juan (must see)

The Fort San Felipe del Morro, or simply El Morro, is a 16th-century citadel on the northwestern tip of the San Juan islet, named in honor of King Philip II of Spain. It was the second military installation, after La Fortaleza, built in what is now known as the Old Town.

The construction began in 1539, authorized by King Charles V, and was finished in 1790. During that period, El Morro had been transformed from a promontory mounted with a cannon to a six-level fortress designed to guard the entrance to the bay of San Juan from seaborne invaders. Many more structures were added to the complex over the next 400 years; the outer walls, built initially 6 feet (1.8 m) thick, were augmented to 18 feet (5.5 m) by the end of the 18th century.

Thanks to El Morro, the Spanish were able to defend Puerto Rico from invasions by the British and the Dutch, as well as pirates. In 1898, following the Spanish-American War, the island changed hands from Spain to the United States. El Morro was actively used as a military installation during the First and Second World Wars.

In 1961, the US Army retired El Morro, passing it on to the National Park Service to use it as a museum. In 1983, El Morro and the walled city of Old San Juan were declared UNESCO World Heritage Sites.

Tips:
A ticket to El Morro allows entry to the Castillo San Cristóbal (and vice versa) for a single fee.